This invention relates to the field of wood burning stoves. More particularly, this invention relates to wood burning stoves incorporating a catalytic combuster for combustion (oxidation) of unburned components (typically carbon monoxide and hydrocarbons) normally present in exhaust smoke.
The use of catalytic combustors is a fairly recent development in the field of wood burning stoves. Catalytic combustors have been incorporated in a number of wood burning stoves sold commercially; and the subject of catalytic combustors has been discussed in various papers, such as the paper entitled "Catalytically Assisted Combustion In Residential Wood-Fueled Heating Appliances" and "Catalytic Combustion In Residential Wood Stoves" presented respectively by J. W. Shelton and R. V. Van Dewoestine at the E.P.A. conducted conference on Wood Combustion Environmental Assessment held in cooperation with The Wood Heating Alliance 1981 International Trade Show and Wood Heating Seminar in New Orleans, La., Feb. 21-24, 1981 and the paper entitled "Catalytic Wood Stoves Utilizing The Corning Wood Stove Combustor (Operation and Design)", January 1982 by F. E. Noll.
It has been known in the field of wood burning stoves that a considerable amount, perhaps as much as 30%, of the chemical energy available from wood fuel leaves a typical wood stove unburned as smoke. This creates not only air pollution problems, but also results in the build up of creosote inside the chimney or the connector from the stove to the chimney. The escape of unburned combustibles is both an economic problem (the cost of heating is increased) and an air pollution problem (which has the potential of becoming of serious magnitude with the increased use of wood burning stoves). The problem of creosote build up is of concern because it can result in dangerous chimney fires, and the creosote can cause corrosion in chimney systems and can create odor problems.
The use of catalytic combustors in wood stoves, as has been proposed in the past, offers the theoretical potential advantages of increases in energy efficiency of the stoves, reduction in pollution, and reduction in creosote build up problems. However, despite apparent indications to the contrary in some literature, practical experience with catalytic combustors has shown the existence of several problems. Practice has shown that high primary combustion flame temperature in the firebox and direct impingement of the flame on the combustor is required in order to fire off the combustor and keep it in full operation. If the primary combustion flame temperature has not been very high and did not impinge directly on the catalytic combustor then the catalytic combustor will not ignite or will not function properly. The direct impingement of high temperature flame heretofore required to light off the catalytic combustor resulted in thermal shock to the combustor, also contributed to burn out of the combustor and high fuel consumption to generate the necessary primary flame temperature to set off the combustor. As a result, combustors (which are expensive) typically burn out or disintegrate in less than one season. This has led to schemes such as flame shields (a metal plate) in front of the combustor to protect against flame impingement; but such shields are counterproductive in that they make it more difficult to light off the combustor. Also, to combat disintegration, elaborate "canning" schemes have been proposed to encase the combustor in a metal case.
Conventional prior art practice for use of catalytic combustors has also required the use of secondary airflow delivered to the catalytic combustor. The secondary air has been necessary in prior systems in order to have sufficient oxygen for combustion in the combustor. However, it is known that secondary airflow can be counterproductive in that too much secondary airflow can quench the catalytic combustor. Furthermore, the secondary air is at a lower temperature than combustion gases, thus aggravating the problem of developing a sufficiently high temperature to set off the combustor.